1. Field of the Invention
The present invention relates to methods and apparatus for forming metal oxide powders. More particularly the invention relates to methods and apparatus for forming metal oxide powders utilizing electric fields to alter their size, shape and porosity.
2. Description of the Related Art
The development of new ceramic materials is sometimes hindered by the inability to reproducibly synthesize high quality starting powders. The ability to control the solid morphology and shape of materials formed from powers is largely dependent upon controlling particle size and size distribution and upon minimizing particle particle interaction. Small particles with a narrow range of particle size distribution are generally desired. Alternatively, a good distribution of particle sizes may include relatively large particles which form the bulk of the material as well as smaller particles used for filling the interstitial spaces between the larger particles during manufacture.
A number of methods have been developed to form monodisperse powders (i.e., particles with a narrow range of particle size distribution and low tendency toward agglomeration) by chemical methods such as the controlled homogeneous precipitation of metal oxides from metal alkoxide by hydrolysis in organic liquid systems. An example of this type of process is U.S. Pat. No. 4,755,369 issued Jul. 5, 1988 to Yoshiharu. U.S. Pat. No. 5,143,711 issued Sep. 1, 1992 to Kluge et al. discloses a process of chemical precipitation of metal-oxy compounds with subsequent thermal treatment. U.S. Pat. No. 5,149,682 issued Sep. 22, 1992 to Spencer et al. teaches the coprecipitation of metal oxides for use in preparing super conducting ceramics. These methods prove to be quite good at producing high purity metal oxide powders. In many cases, however, the conditions which are favorable for silicon and metal alkoxide hydrolysis and the subsequent silicon oxide and metal oxide precipitation are not amenable to minimizing particle-particle interactions and consequently, the powders form agglomerates and become polydispersed. It is sometimes advantageous to control aggregation of particles to produce aggregates with different shapes, sizes and morphologies. This can be done by imposing an electric field on the reacting solution. Other attempts to produce monodispersed metal oxide powders using the technique of metal alkoxide hydrolysis have involved the use of mechanical stirrers to disperse the aqueous phase in the organic liquid system. These techniques are energy intensive and generally do not produce metal oxide powders with the desired size distribution.
Other non-electrolytic methods include spray drying of bauxite clays to produce microspheres as taught in U.S. Pat. No. 5,175,133 issued Dec. 29, 1992 to Smith et al., and metal coating of ceramic powder particles by chemical absorption taught in U.S. Pat. No. 5,102,592 issued Apr. 7, 1992 to McCauley et al.
Electrolytic processes for producing metal oxide powders include the electrodeposition of a metal oxide film on an electrically charged plate using a constant current as taught in U.S. Pat. No. 4,818,352 issued Apr. 4, 1989 to Inaba et al., and precipitation of metal particles using a constant DC current and a constant pH as taught in U.S. Pat. No. 4,670,114 issued Jun. 2, 1987 to Beer et al., for example.
U.S. Pat. No. 5,112,433 issued May 12, 1992 to Dawson et al. teaches the formation of perovskite particles using an electrolytic solution and a controlled pH. U.S. Pat. No. 5,166,130 issued Nov. 24, 1992 to Enomoto et al. teaches the production of an electrically conductive ceramic film wire. Whereas U.S. Pat. No. 4,810,339 issued Mar. 7, 1989 to Heavens et al. teaches a slurry of metal oxide powders layered onto a substrate using electrophoretic deposition.
U.S. Pat. No. 5,122,360 issued Jun. 16, 1992 to Harris et al. teaches a method of preparing metal oxide powder wherein a first solution which is substantially organic has delivered into it as drops a second solution substantially immiscible in the first solution. The drops of the second solution are atomized by a pulsed electric field forming "micro-drops" of the second solution. Reagents in the first solution diffuse into and react with reactants in the micro-drops of the second solution forming metal hydroxide or oxalate particles which are then recovered. This reference teaches the criticality of utilizing solution having two liquid phases whereas the present invention utilizes only a single phase liquid solution.